Polyolefins are considered non-polar polymers, used for many purposes such as, filaments, tapes, fibers, films, etc. One of the main field of polyolefins use is in the manufacturing of composite materials.
As known, a main problem encountered with production of composites, is the non-polarity of the polyolefins, which causes difficulties in obtaining a good adhesion between the non-polar polyolefins and the extraneous materials, such as plastic or resin, which generally are more polar than the polyolefins.
Ultra-high molecular weight polyethylene (hereinafter referred to UHMWPE) is a linear high density polyethylene (HDPE) with a molecular mass in the range of between 1.times.10.sup.6 to 16.times.10.sup.6. Its very high molecular mass imparts an exceptional impact strength and abrasion resistance as well as special processing characteristics. These unusual properties preclude the use of conventional extrusion and moulding techniques.
Fibers made from this type of polyethylene are characterized by their high modulus and strength, light weight and high energy dissipation in comparison with other polymer fibers. However, the standard extrusion and molding techniques for obtaining fibers of UHMWPE are not applicable.
A main deficiency of UHMWPE fibers in its use as reinforcement in composites materials, is their relatively poor adhesion to the matrix in a composite and their chemical inertness as mentioned in a recent review (D. N. Hild et al, J. Adhesion Sci. Technol. 6, p. 879, 1992). As known, the stress-transfer ability of the fiber-matrix interface and accordingly the mechanical properties of such composites are greatly affected by the level of the fiber-matrix adhesion. The compatibility between the thermoplastic UHMWPE fiber and the thermoset resins is also limited due to the non-polar property of the polyethylene.
Composites of polyethylene and UHMWPE, obtained by hot compression molding at a temperature between the melting points of the fibers and the polyethylene matrix, were found to comprise a uniform transcrystalline layer of the polyethylene melt on the UHMWPE fiber surface (Teishev et al. J. Appl. Polym. Sci., 50, 1993, p.503).
The European Patent Application Number 313,915 is suggesting a process to improve the adhesion of polyolefin objects to polar polymer matrices. The process involves a treatment of the surface of polyolefin objects obtained from a solution or melt, having a molecular weight of at least 400,000 g/mol, by its immersion into a solvent at a temperature above that of the polyolefin dissolution. A most preferred solvent which is suggested is xylene. It is claimed that the treated objects according to this process retain their adhesive strength to the polar matrices for a long period of time.
In the U.S. Pat. No. 4,563,392, it is described a method for obtaining a coated polyolefin fiber having an increased adhesion to matrix materials. According to this method the multifilament fiber having a molecular weight of above 500,000, is coated with a polymer possessing the ethylene or propylene crystallinity, said coating being between 0.1% to about 200% by weight of the fiber.
According to a very recent paper by Roger S. Porter et al (Polymer, 35, 23, 1994, p.4979-84), high-modulus and high-strength UHMWPE bars or films are obtained, by a two-stage drawing technique: by direct compaction followed by calendering at a temperature below the melting point.
In another recent paper by B. L. Lee et. al. (Journal of Composite Materials, Vol. 28, No. 13.1994, p. 1202-26), there are described tests which were carried out on polyethylene fiber-reinforced composites and examined under ballistic impact loading.
The above brief review, illustrates that the subject of fiber and matrix of UHMWPE composite material is indeed considered an interesting problem which indicates that it requires more investigation.
It is an object of the present invention to provide novel composite materials based on fiber and matrix of UHMWPE. It is another object of the present invention to provide a process for obtaining composite materials based on UHMWPE having improved mechanical properties. It is yet another object of the present invention to provide a process for obtaining a material based on fibers of UHMWPE with improved adhesion property to a polymer matrix.